Interpretive Summary: Precipitation is a dominant input in agricultural production systems, and a better understanding of the changing nature of long-term precipitation can assist farmers and ranchers in making strategic management and production decisions. The objective of this research is to identify variations of average annual precipitation over 10 and more years, and corresponding changes in daily rainfall amounts and number of rainy days. Within the 1897-1999 period of record, an increase in precipitation in the Great Plains during the last two decades of the 20th century was identified. In central Oklahoma the bimodal seasonal precipitation pattern changed little during this period, yet a few late spring, early summer and fall months captured a larger portion of the precipitation increase. A slight decrease in summer precipitation was also observed. Changes in daily rainfall amount and number of rainy days were investigated for the Kingfisher location in central Oklahoma. It was found that the annual precipitation increase at the end of the 20th century could be attributed mostly to more rainy days and not to higher and more frequent extreme precipitation events. This investigation suggests that winter wheat production and grazing at the end of the 20th century in central Oklahoma may have benefited most from this increased precipitation.

Technical Abstract:
Farming and ranching in Oklahoma are subject to climate and weather fluctuations that produce annual variation in forage and crop yields. The recognition of such fluctuations may provide opportunities for farmers and ranchers to incorporate climate information into their planning and management decisions, and to assist them in moving towards more profitable production systems. Within the 1897-1999 period of record, an increase in precipitation in the Great Plains during the last two decades of the 20th century was identified. In central Oklahoma the bimodal seasonal precipitation pattern did not change during this period, yet a few late spring, early summer and fall months captured a larger portion of the precipitation increase. A slight decrease in summer precipitation was also observed. Changes in daily rainfall amount and number of rainy days was investigated for the Kingfisher location in central Oklahoma. It was found dthat the annual precipitation increase at the end of the 20th century coul be attributed mostly to more rainy days and not to higher and more frequent extreme precipitation events. The changes in daily precipitation statistics were found to be most valuable for use by computer models to track changes in runoff, soil erosion, and productivity over long duration.